Laser printers and copying machines typically use electrophotographic techniques to transfer dry toner particles to a rotating drum or to a sheet of paper by electrostatic attraction. After the toner is transferred to the paper, the paper is then heated to melt the toner so that the toner permanently adheres to the paper. There are many well-known types of these devices, and details of their operation need not be presented here.
Usually, the dry-particle toner is replenished by replacing a toner cartridge in the printer. The toner cartridge is typically a plastic receptacle containing the dry toner.
FIG. 1 illustrates one of many prior art toner cartridges, which is a replaceable unit in electrophotographic printers, such as laser printers and copiers. The techniques of the present invention may be used with any type of known toner cartridge. Toner cartridge 10 includes a plastic housing 12, which is normally opaque but is shown as transparent for purposes of this disclosure.
Powdered toner 14 is shown contained within housing 12. For monochrome electrophotographic printing devices, toner 14 is typically a fine, black, resinous powder. The toner is either deposited directly on charged paper or transferred from a charged surface, such as a drum, belt, or roller, to ordinary paper, then fused to the paper by heating. Toner 14 can be any known toner and need not be further described herein.
Conventional toner cartridges, such as cartridge 10, include a stirring rod 16, which may take many forms such as a rotating bar or paddle which slowly agitates the toner 14 near the bottom of a sloped trough 18 to prevent clumping of the toner and to provide even feeding of the toner into physical elements of the printing process. A typical rotation speed of stirring rod 16 is 10-30 rpm in a 24 page-per-minute printer.
Cartridge 10 includes a developer roller 20 that attracts a thin layer of toner 14 on its surface and transfers the toner particles to a photoreceptor drum (not shown) in the printer. The photoreceptor drum is selectively charged using a laser or other technique such that toner 14 only adheres to the drum in selected areas. The toner on the drum is then transferred to a sheet of paper. The paper is then heated to fuse the toner to the paper.
In other existing toner cartridges, such as cartridge 22 in FIG. 2, along with a stirring rod 24 is a primary charger roller 26 and an organic photoconductor (OPC) roller 28 proximate to the charger roller. The primary charger roller 26 charges the OPC roller 28, and a laser selectively exposes the OPC roller 28 in a pattern that produces the desired recorded image. A developer roller 20 supplies a thin layer of toner to the OPC roller 28 in the selected areas. The toner on the OPC roller 28 is then transferred to a sheet of paper to record the image onto the paper. The paper is then heated to fuse the toner completing the process. An exemplar of this toner cartridge is the C3909A LaserJet cartridge for the Hewlett Packard 5SiMX LaserJet printer.
A metal wire 32 runs proximate to the developer roller 20. A sensor (not shown) connected between wire 32 and the developer roller 20 senses the capacitance between wire 32 and the developer roller 20. When the quantity of toner is depleted to the extent that it exposes wire 32, the detected capacitance undergoes a significant change, and this is used to generate an indication to the user that the toner level is low.
Other elements may also be incorporated in toner cartridges.
It is important that an indication of toner quantity be available to a printer user either for a local or networked printer. This indication can be presented as a display on the printer or a display or message visible to the user whenever the printer is invoked by an application program. Knowledge that the remaining quantity of toner is inadequate for a printing task is of significant value to a user and to service personnel. Some solutions in the current art, in addition to that described with respect to FIG. 2, include providing a toner cartridge with a window through which the user or a photosensor may observe toner quantity, incorporating electrodes into the toner cartridge to detect a threshold quantity of toner, or other techniques which require a modification to the toner cartridge such as floats, paddles, or other physical sensors known in the art operating in contact with the toner. Modifying existing toner cartridges to include a means for sensing the toner quantity adds cost to each cartridge. In addition, all methods currently in practice do not operate over the full range of toner quantity from full to empty and offer poor accuracy.
What is needed is a more economical and accurate technique for measuring the toner quantity in a toner cartridge.